An increasing world population leads to a continually increasing amount of refuse. Additionally, a increased level of civilization appears to generate an increased amount of refuse on a per capita basis. Both factors in combination lead to mounting pressure to devise methods of waste disposal which are economically, energetically, and environmentally sound.
In recent years, especially in urban areas, the increased demand for usable land and other concerns has caused one to turn from a landfill as the major mode of refuse disposal to other options, especially the use of raw refuse as an energy source. One variant of the latter is the mass burning approach, where all the refuse in its raw state is burned without any preliminary treatment such as separating the noncombustible from the combustible material. Quite briefly, in this method solid wastes, for example, raw garbage, are dumped into storage where it is homogenized and dried to some degree. Refuse from the storage area is fed into a combustion zone where the heated gases often are used to generate steam. Flue gases then pass from the combustion zone to a separation zone, often an electrostatic precipitator, where dust and ash are removed. The ash so removed from the flue gas, called fly ash, is then mixed with the ash collected in the combustion zone, called bottom ash, and the combined ash used for landfill.
In certain states which regulate ash under RCRA, ash is currently either managed as a solid or a hazardous waste depending on the lead (Pb) and cadmium (Cd) leachability as revealed by the Toxicity Characteristics Leaching Procedure (TCLP) extraction test defined by the Resource Conservation and Recovery Act (RCRA). The TCLP extraction test predicts the effect of decomposing organic material in a landfill on wastes being considered for landfill waste disposal. When organic material decomposes, acetic acid is formed. Thus, in the TCLP extraction test, both water and acetic acid are used to extract leachable heavy metals, such as lead and cadmium from the waste, which are then quantified.
It is well known that some of the more volatile compounds of certain metals tend to accumulate in the fly ash. Especially where the latter is to be used as landfill with decomposing solid wastes, leaching of toxic metals, especially cadmium and lead, constitutes a potential hazard to the ecosystem, for example, both surface water supplies and aquifers. As such, fly ash often fails the TCLP regulatory limit of 5.0 mg/l for lead and 1.0 mg/l for cadmium. I have found that bottom ash contains little or no leachable cadmium because cadmium with a low vaporization temperature (about 600.degree. F.) is efficiently volatilized from bottom ash at high grate temperatures (1600.degree.-2500.degree. F.). Bottom ash will occasionally fail the TCLP test in regards to leachable lead, due primarily to lead's higher vaporization temperature (1600.degree. F.) in conjunction with the operating grate temperature at the time. If the TCLP test is failed, the fly and/or bottom ash may be handled as a hazardous waste depending on state regulations. Such a designation entails additional handling procedures and special disposal sites. Hazardous materials are always more expensive to manage than nonhazardous wastes.
One process for treating fly ash alone or in combination with bottom ash to immobilize leachable lead and cadmium therein is described in U.S. Pat. No. 4,737,356. The treatment involves contacting the dry ash with a source of water-soluble phosphate and with an alkaline source of calcium to immobilize the lead and cadmium in the ash. As noted earlier, bottom ash typically contains little leachable cadmium. Thus, the combining of the bottom ash with the fly ash artificially reduces the cadmium levels of the mixture. However, this results in greater quantities of combined ash requiring drying and treatment, thereby increasing equipment sizes and associated costs and handling requirements. Thus, the bottom ash is occasionally and preferably disposed of separately from fly ash.
Thus, there is a need for a method to safely dispose of bottom ash or to inexpensively render it safe for disposal or for use as an aggregate or building material without concern of leaching of lead.
It is noted that both untreated fly ash and bottom ash are often regarded as hazardous waste products in certain states which require TCLP. Thus, it is highly desirable to reduce the amount of lead leached from bottom ash containing leachable lead to an amount below the TCLP regulatory level specified by the EPA. It would also be desirable to have a process for treating bottom ash that could be retrofitted into existing treatment facilities without substantial capital expense or change in the operations of the process to allow for disposal as a solid waste or reuse. It would further be desirable to have a process for treating bottom ash without first having to dry it or to mechanically process it, i.e., provide a wet in situ treatment for bottom ash.
The invention herein is a solution to this problem. More specifically, it is a method of treating bottom ash and mixtures containing bottom ash, so as to reduce the amounts of lead leached therefrom to a level below the TCLP regulatory level specified by the EPA. Stated differently, the invention herein is a method of immobilizing, or insolubilizing, lead in bottom ash. The method is convenient, quite simple, very efficient, applicable over a wide pH range, and relatively low cost. An important advantage of the method of the present invention is that the bottom ash may be wet or dry. Another important advantage of the method of the present invention is that the method can be performed in a totally "enclosed" environment, thereby exempting the practice of the present invention in this manner from RCRA Part B permitting. The method is, therefore, commercially extraordinarily attractive as well as being environmentally beneficial.